Coupled nitrogen and phosphorus dynamics in a forested headwater stream.
Scientists at Oak Ridge National Laboratory (ORNL) examined the effects of single and dual nitrogen and phosphorus additions on nutrient cycling in a co-limited (i.e., for nitrogenand phosphorus) headwater stream (Walker Branch, Tenn.).
There is a growing need to investigate coupled biogeochemical cycles, especially in ecosystems that may be co-limited (e.g., for nitrogen and phosphorus). This novel research approach used two nutrient addition techniques to investigate coupled nitrogen and phosphorus cycling in stream reaches and may be applied to other elemental cycles and environmental settings.
Nitrogen and phosphorus can limit autotrophic and heterotrophic metabolism in lotic ecosystems, yet most studies that evaluate biotic responses to colimitation focus on patch-scale (e.g., nutrient diffusing substrata) rather than stream-scale responses. In this study, ORNL scientists evaluated the effects of single and dual nitrogen and phosphorus additions on ambient nutrient uptake rates and saturation kinetics during two biologically contrasting seasons (spring and autumn) in Walker Branch, a temperate forested headwater stream in Tennessee, USA. In each season, they used separate instantaneous pulse additions to quantify nutrient uptake rates and saturation kinetics of nitrogen (nitrate) and phosphorus (phosphate). The team then used steady-state injections to elevate background stream water concentrations (to low and then high background concentrations) of one nutrient (e.g., nitrogen) and released instantaneous pulses of the other nutrient (e.g., phosphorus). The researchers predicted that elevating the background concentration of one nutrient would result in a lower ambient uptake length and a higher maximum areal uptake rate of the other nutrient in this co-limited stream. Their prediction held true in spring, as maximum areal uptake rate of nitrogen increased with elevated phosphorus concentrations from 185 µg m–2 min–1 (no added phosphorus) to 354 µg m–2 min–1 (high phosphorus). This pattern was not observed in autumn, as uptake rates of nitrogen were not measurable when phosphorus was elevated. Further, elevating background nitrogen concentration in either season did not significantly increase phosphorus uptake rates, likely because adsorption rather than biotic uptake dominated phosphorus dynamics. Laboratory phosphorus sorption assays demonstrated that Walker Branch sediments had a high adsorption capacity and were likely a sink for phosphorus during most pulse nutrient additions. Therefore, it may be difficult to use coupled pulse nutrient additions to evaluate biotic uptake of nitrogen and phosphorus in streams with strong phosphorus adsorption potential. Future efforts should use dual nutrient addition techniques to investigate reach-scale coupled biogeochemical cycles [C-N-P, and other elemental cycles, such as iron (Fe), molybdenum (Mo)] across seasons, biomes, and land-use types and over longer time periods.
BER Program Manager
Terrestrial Ecosystem Science, SC-23.1
Natalie A. Griffiths
Oak Ridge National Laboratory
Oak Ridge, TN 37831
This research was part of the long-term Walker Branch Watershed project at Oak Ridge National Laboratory (ORNL) and supported by the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy (DOE) Office of Science. ORNL is managed by UT-Battelle, LLC, for DOE under Contract No. DE-AC05-00OR22725. Appreciation is extended to T.V. Royer for partial laboratory support and to Indiana University’s School of Public and Environmental Affairs for supporting LTJ’s time.
Griffiths, N.A., and L.T Johnson. “Influence of dual nitrogen and phosphorus additions on nutrient uptake and saturation kinetics in a forested headwater stream.” Freshwater Science 37(4), 810–825 (2018). [DOI:10.1086/700700]
Data citation: Griffiths, N.A., and L.T. Johnson. 2018. Walker Branch Watershed: Effect of Dual Nitrogen and Phosphorus Additions on Nutrient Uptake and Saturation Kinetics, 2011–2012. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee, U.S.A. [DOI:10.25581/ornlsfa.015/1484490]
Data are available on the ORNL TES SFA website: https://tes-sfa.ornl.gov/node/80#WBW_new
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